1. Field of the Invention
The invention generally relates to a network node. More specifically, the present invention relates to a network node with optical add/drop modules.
2. Description of the Prior Art
A network node for use in a ring network is generally disclosed in German Patent Document No. DE 19731494 C4. The ring network provided for transmitting data in the wavelength division multiplex method has a working device, a protection device and a number of network nodes with add/drop modules. The distinctive feature of this known ring network, the transmission capacity of which is to be utilized to an optimum extent, consists in using only a single protection channel which can be accessed by all terminals, i.e., being able to send data and to receive data.
As disclosed in European Patent Document No. EP 0 847 158 A2, a network node with optical add/drop modules for bidirectional networks is also known in which a separate module OADM1 and OADM2, respectively, is provided for each of the two directions of transmission, in which in each case an add function and a drop function is combined for the respective direction of transmission. The known arrangement is neither provided for a ring network nor does it contain a protection device.
As disclosed in European Patent Document No. EP 0 892 524 A2, a network node with optical add/drop modules in a unidirectional ring network that has a number of ring lines is also known. In the case of a “single point of failure” in the network, the signals are diverted via other links of another ring line which are not faulty.
It is also known as disclosed in European Patent Document No. EP 0 729 247 A2 to guarantee the transmission capacity in bidirectional ring networks, for example in the case of a disturbance, by transmitting on each fiberglass waveguide, in addition to the operating signal, a replacement signal of different wavelength. In the case of a disturbance, i.e., in the case of an interruption of the connection between two network elements, the operating signal between the network elements of the ring network affected by the failure of the connection is switched to the replacement signal, and thus to the second wavelength, thereby maintaining the full transmission capacity. The network elements provided are add/drop multiplexers which are arranged in parallel with existing optical interfaces at in each case additional optical interfaces. These additional interfaces operate in a different wavelength range in contrast to the existing optical interfaces. Operating signal and replacement signal of an optical transmitter are combined in a suitable manner by optical multiplexers at the output of each network element and transmitted to the receiving site by the respective optical fiber waveguides. At the receiving site, an optical demultiplexer separates the two optical signals of different wavelengths and supplies them to the further processing stages.
In purely optical wavelength division multiplex networks, some of the optical connections will be purely of a static nature but some others will have short lifetimes. To optimally utilize the network capacities, there exists a need for automatically reconfigurable and purely optical add/drop multiplexers. In this regard, the reconstruction of existing systems may necessarily need to take place with continuing traffic, i.e., careful protection of the continuing traffic without optical “single point of failure” is necessarily required.
In addition, the ability to implement not only traffic links via a central hub-type network node but any point-to-point connections may necessarily be needed. A typical traffic pattern in future two-fiber ring networks will necessarily include a forward return connection in different fibers on the same link. In this regard, the corresponding protection connection leads via a different route in two fibers, thus providing for a reliable standby connection in the case of a fiber break.
Due to the minimization of production costs in metropolitan ring networks, great differences in level must be expected in the individual wavelength channels.
In this regard, the static add/drop multiplexers in purely optical wavelengths division multiplex rings will necessarily need to be expanded into dynamically remotely and configurable add/drop multiplexers, thus an “in-service” expansion should be possible.
Static optical add/drop multiplexer network elements have only been on the market for a very short time (CAMBRIAN, CIENA, OSICOM, LUCENT, SIEMENS 04/99), where commercial remotely configurable systems are expected for the year 2000. The remote configurability of optical add/drop multiplexer modules has currently only been implemented by way of expensive optical circuit technology (i.e., wavelengths division multiplexers, demultiplexers and space division switching matrices).
However, no corresponding arrangements for setting up remotely configurable protected 1+1 connections have been reported although the use of fiber gratings in add/drop modules is described in U.S. Pat. No. 5,748,349.
An object of the present invention, therefore, relates to protecting the transmission of a WDM signal from an interruption of the line when, for example, modules are exchanged in a ring network.